Question
Answer and Explanation
The inability to perform incremental writes when changing encryption methods, especially on data at rest, stems from the fundamental principles of cryptographic transformations. Here's a breakdown of why this is often the case:
1. Block Ciphers and Data Dependencies:
Many encryption algorithms, like AES (Advanced Encryption Standard), are block ciphers. They operate on fixed-size blocks of data (e.g., 128 bits for AES). When you change the encryption key or algorithm, every block needs to be re-encrypted using the new parameters. Because each block’s encryption depends not only on its own contents but, in many common modes of operation (like CBC, Cipher Block Chaining, or GCM, Galois/Counter Mode), on the encryption of the prior block, you cannot selectively re-encrypt only one part of a file. You often need to decrypt and then re-encrypt the whole set of the data blocks in the correct order.
2. Integrity and Authentication:
Modern encryption modes, like GCM, include an authentication tag or message authentication code (MAC) along with the encrypted data. This is crucial to ensure data integrity. Changing the encryption requires the MAC to be recalculated. The MAC depends on the whole encrypted data. If we could just replace a single encrypted block, the authentication would be invalid.
3. Chaining Modes of Operation:
Modes like CBC use chaining, where each block's encryption is influenced by the previous block's ciphertext. This means if you change the encryption key or algorithm, even if you could modify just one encrypted block, you must re-encrypt all subsequent blocks as well to maintain correctness. Otherwise you would introduce data corruption.
For example, in CBC mode, if you only re-encrypt block n, you’d need to decrypt blocks n, n+1, n+2, etc, then re-encrypt from n onwards.
4. Key Derivation and Initialization Vectors (IVs/Nonces):
Often, encryption keys are not used directly but instead derived from a master key plus a unique salt or IV. When you change the encryption process, these parameters also usually change. The entire encryption process relies on starting from correct initial conditions. Using new IVs implies changing the entire ciphertext from the point at which the new IV is used.
5. Practical Limitations:
- Performance: While theoretically you might think some minor changes are possible, the computational overhead to re-derive parts of encryption is typically not optimized. Processing the entire data is often much more straightforward and efficient in code implementation.
- Complexity: Allowing incremental changes in encryption would greatly complicate implementation and increase the risk of error. This could introduce new vulnerabilities. It’s often safer to re-encrypt the entire data set in a controlled manner.
6. Example scenario:
Suppose you have a database encrypted with AES-256 in CBC mode. Now you want to switch to AES-256 in GCM mode. You can not simply re-encrypt selected fields of your database. If you change only a field, your previous encrypted data will not match the expected structure, and you will not be able to decrypt the fields that were not converted or those that depend on them.
In summary, the lack of support for incremental encryption changes isn't usually an oversight but a result of fundamental cryptography principles that prioritize security and correctness. Instead of incremental changes, the general best practice is to perform a full re-encryption with the new parameters when changing encryption methods. This approach avoids all the complications described above and makes sure your data remains secure and consistent.